The present invention is particularly well suited for use in connection with large rectangular balers of the type illustrated in U.S. Pat. No. 4,106,267, which are capable of producing bales weighing one ton or more. As big balers of that type are driven along a crop windrow, towed behind a forwardly disposed tractor, the materials are picked up directly beneath the baler and are fed up into the overhead bale chamber in successive charges for subsequent compaction by a fore-and-aft reciprocating plunger in the chamber. Prior to the baling operation, the hay will normally have been gathered into long windrows, sometimes in side-by-side, so-called "double windrows", so that the baler is simply towed along the windrows straddling the same with its ground wheels on opposite outboard sides of the windrows. With the baler aligned with the windrows in this way, the operator can be fairly assured that all of the windrowed material is being picked up and fed into the baler.
However, it is not possible to prepare the windrows in such a manner that they are always of perfectly uniform volume across their widths. Thus, even though the operator may be driving straight down the windrow, it is possible that more material may be going into one side of the baling chamber than the other. And it is also not possible for the operator to at all times maintain the baler perfectly positioned on the windrow. Consequently, one side of the finished bale may contain a greater volume of materials than the other, which can have an adverse effect on the shape of the bale and total payload. In extreme conditions where the bale is seriously misshapened, it may even be difficult for the binding twines to remain in place around the bale, and subsequent hauling, stacking, and other handling may be substantially impaired. Moreover, there is no way for the operator to realize that the uneven loading problem exists until the deficient bale has been completely formed and discharged from the machine, at which point it is too late to correct the problem with respect to that particular bale. Furthermore, uneven loading in the baling chamber can result in high stress on critical parts of the drive train for the plunger.